Hold-down system



Aug. 13, 1940. w. FERRls Plomb-Down SYSTEM Original Filed Aug. 3, 1956 .s sheets-sheet 1 Aug. 13, w FERRS r A HOLD-Down SYSTEM Original Filed Aug. "'3, 1936 3 Sheets-Sheet 2 75am? Conro/ 7 INVENTDR 34 6 G p l WALTER F'i-:Rms 44 BY 2 AT1-mma@ Aug. 13, 41940. w. FERR@ y'2,211,692

HOLD-DOWN SYSTEM Original Filed Aug. 3, 193e :s sheets-sheet s 29 /37 l[30 la; /31`9 /4/ 27 Fig. 1 5.

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O 5o l V5 INVENTCIR 5 WALTER Farms EY K 0 `5 v a TTDRNEY.

Patented Aug. 13, 1940 UNITED STATES PATENT OFFICE HOLD-DOWN SYSTEM Walter Ferris, Milwaukee, Wis., assignor to The Oilgear Company, Milwaukee, Wis., a. corporation of Wisconsin 15 Claims.

This application is a division of application Serial No. 94,034 filed August 3, 1936 and now abandoned.

The invention relates to hydraulic mechanisms 5 Aof the type having a 'plurality of reciprocating hydraulic motors for exerting holding or pressing forces at spaced apart points.

An object of the invention is to provide a mechanism of this character in which a plurality of reciprocating hydraulic motors are operated by motive liquid supplied thereto from a single source and controlled by mechanism which enables each motor to operate at high speed until it is exerting a holding or pressing force and then to automatically maintain the holding or pressing force within narrow limits without affecting the operation of or the force exerted by any other of the motors.

Another object is to provide a control which will cause the pistons of a plurality of reciprocating hydraulic motors to move in unison.

Other and more specific objects and advantages will appear from the description hereinafter given of a hold-down system in which the invention is embodied, the system being shown schematically in accompanying drawings in which the views are as follows:

Fig. 1 is a diagram showing the hydraulic circuit and the functional relations of the several parts of a hold-down press having four sections each of which includes a reciprocating hydraulic motor for operating a pressing member or platen.

Fig. 2 is a diagram showing on a larger scale one section of the press with the valves thereof in positions to direct liquid to the hydraulic motor to advance the platen at high speed.

Fig. 3 is a view of part of the mechanism shown in Fig. 2 but with the parts in the positions occupied when a holding pressure is being maintained 40 in --the cylinder of the motor.

Fig. 4 is a view showing the by-pass and reversing valves in the positions occupied to effect reversal of the hydraulic motor.

Fig. 5 is a view showing the by-pass valve in 45 position to by-pass the pump and thereby stop the.. press.

Fig. 6 is a detail view.

.For the purpose of illustration the invention has been shown applied to a press having four sections X, Y, Z and W but it is equally applicable to a press having a diilerent number of sections. Since the press frame and the details of construction form no part of the present invention, the same have been omitted from the drawings in order to avoid complicating the views, it being suilicient to state that lthe hydraulic motors and the control elements are suitably xed in stationary positions.

Liquid for operating the press is supplied by a pump 30 which has the characteristics of delivering liquid at a predetermined maximum rate until pump pressure reaches a predetermined maximum and then automatically reducing its stroke until it is delivering just sulcient liquid to maintain that maximum pressure substantially constant. Pump 30 may be of the general type shown in Patent No. 2,074,068, and it may be provided with a control of the type shown in Patent No. 2,080,810.

Since pump 30 and its control are fully illustrated and described in the above patents, only a brief description thereof will be given herein. The pump has its pistons and cylinders arranged in a rotatable cylinder barrel 3| which is journaled upon a stationary valve shaft or pintle 32 through which liquid flows to and from the cylinders. 'I'he pump pistons are reciprocated by having the outer ends thereof in contact with an annular reaction surface 33 shown as being arranged upon the inside of a slide block or thrust member 34 which is closely fitted in the pump casing 35 and shiftable therein transverse to the cylinder barrel axis to vary pump displacement. In practise, however, reaction surface 33 is formed in a thrust member which is arranged within side block 34 and rotatively supported thereby. Slide block 34 is urged in a direction to increase pump displacement by two springs 36 and is adapted to be moved in the opposite direction to decrease pump displacement by liquid supplied to a cylinder 31 which is carried by casing 35 and has a piston 38 fitted therein and connected to slide block 34.

Liquid for operating piston 38 is supplied by the pump itself under the control of a control mechanism 39 which is connected by a channel 40 to cylinder 31 and by a channel 4l to a supply channel 42 through which pump 30 delivers liquid to all four sections of the press. Control mechanism 39 has not been shown in detail as it is fully illustrated and described in Patent No. 2,080,810. It is deemed sucient to state herein that control mechanism 39 permits liquid to escape from cylinder 31 when pump pressure is below a predetermined maximum so that springs 36 can shift slide block 34 toward the left to increase pump displacement, and that it permits liquid to flow from channel 42 into cylinder 31 and shift slide block 34 toward the right to decrease pump displacement upon pump pressure exceeding a predetermined maximum. When cylinder barrel 3| is rotated and the axis of reaction surface 3l is offset from the axis of cylinder barrel Il, pump will discharge liquid into supply channel 42 and be supplied with liquidthrough an intake channel 43 from a reservoir 44 to which liquid is returned from the circuit through a return channel 45.

Pump 30 may draw its entire supply of liquid from reservoir 44 through channel 43 or the liquid required by pump 38 may be suppliedin whole or in part by an auxiliary pump such as a gear pump 48 which draws liquid from reservoir 44 and delivers it through a channel 41 into channel 43 in which case channel 43 is provided with a check valve 48 to prevent liquid from escaping through the open end thereof and channel 41 has connected thereto a relief valve 48 through which liquid discharged by gear pump 48 in excess of requirements is exhausted, particularly when pump 30 is delivering only sufllcient liquid to maintain a holding pressure in the system.

For the purpose of illustration, the press has been shown provided with a hand operated control lever 50 by means of which the press may be started, stopped or reversed. Lever 50 is pivoted intermediate its ends upon a stationary pivot 5| and is connected to a reversing switch 52 and to the plunger 53 of a by-pass valve 54 sov that it may operate switch 52 and by-pass valve 54 simultaneously.

Reversing switch 52, which functions to reverse certain control motors to be presently described, is connected to those motors by suitable conductors arranged in a conduit 55 and to a power source (not shown) by suitable conductors arranged in a conduit 56.

Plunger 53 of by-pass valve 54 is provided with two spaced apart heads or pistons 51 andi 58 which are closely tted in a bore 59 formed in the valve casing. Bore 59 is connected at both of its ends to return channel 45 by a branched channel 60 and it is connected intermediate its ends to channels 42 and- 45, respectively, by two channels 6i and 62.

When plunger 53 is in either of its end positions as shown in Figs. 2 and 4, one or the other of channels 6i and 62 is blocked by one or the other of pistons 51 and 58 so that pump 30 may deliver liquid to the hold-down motors to be presently described and, when it moves from one end position to the other end position, channels 6| and 62 are momentarily in communication with each other, as shown in Fig. 5,l so that the liquid discharged by pump 30 is momentarily bypassed through channels 42 and 6I, bore 59 and channels 62 and 45 to reservoir 44 and the pressure in channel 42 momentarily drops to a very low value for a purpose to be presently explained. In order to limit the speed at which plunger 53 may be moved near the limit of its movement in either direction, valve 54 is provided with a dashpot on each end thereof. As shown, plunger 53 has dashpot plungers 83 and 64 arrangedA upon opposite ends thereof and adapted, respectively, to enter dashpots 65 and 66 arranged at opposite ends of bore 59 and carried by the by-pass valve casing. Dashpots 65 and 68 are connected, respectively, to channel 60 through check valves 61 and 68.which permit liquid to flow from channel 60 into the dashpots but prevent liquid from escaping from the dashpots into channel 68.

The arrangement is such that liquid may ilow freely from one end of bore 58 to the other end thereof through channel Il and from channel 88 into the rear dashpot during the entire movement of plunger 88 in either direction. Consequently, plunger Il may be moved at high speed far enough to open communication between channels 8I and 82 and then the forward dashpot plunger will enter the other dashpot and decelerate plunger I3 sufficiently to maintain communication between channels Il and 82 long enough to permit the pressure in channel 42 to drop to a very low value.

The liquid supplied by pump 8l through channel 42 is employedto operate a plurality oi hydraulic motors which may be of the same size or of diiierent sizes and controlled by substantially identical control mechanisms. Since each motor and its control mechanism functions independently of any other motor and the control mechanism thereof, each motor and its control mechanism may be considered as an independent press section and the press may be divided into as many such sections as there are motors, four being shown and indicated by the reference letters X, Y, Z, and W.

Since the four sections are substantially identical, details of section X only have been shown and a description of that section will sumce for all sections, corresponding parts of the other sections having been indicated by corresponding reference characters.

Referring now more particularly to Fig. 2, section X contains a hydraulic motor consisting primarily of a stationary cylinder 18 and a piston 1I which is fitted in cylinder 10 and connected by a piston rod 12 to a pressure member or platen 13. The motor may press platen 13 against a stationary die block or abutment 14 or urge it toward abutment 14 to press therebetween the material to be operated upon.

'I'he direction of movement of piston 1i is controlled by a reversing valve 15 the plunger 18 of which has two spaced apart heads or pistons 11 and 18 formed thereon and closely fitted in the valve casing to control the flow of liquid therethrough. 'I'he reversing valve casing is connected at its extreme ends by two channels 19 and 88 to returny channel 45, and it is provided intermediate its ends with two distributing ports 8| and 82 and with an inlet port 83 which is arranged between the two distributing ports and connected to supply channel 42 by a channel 84.

Port 8l communicates with one end of a channel 85 the other end of which is connected through a check valve 88 and` a resistance valve 81 to one end of a-channel 88 the other end of which is connected to the lower end of cylinder 18. Check valve 86 permits liquid to flow freely to the lower end of cylinder 10 to raise piston 1i, but it forces liquid ejected from the lower end of the cylinder 10 to flow through resistance valve 81 which oiers a low resistance to the ilow of liquid therethrough and thereby prevents piston 1I from moving downward when the press is idle.

Port 82 is connected by a channel 90 to a pressure responsive valve 9| which either directs liquid freely to'the upper end of cylinder 'lll to move piston 1I downward or directs liquid thereto to maintain a holding pressure therein through a pressure control valve consisting primarily of two oppositely opening resistance valves 92 and 93.

In practice, the two resistance valves are arranged in alignment in a single casing or in connected casings and are adjusted by a single element but, for the purpose of illustration, they while rotation of member 94 in the opposite direction will decrease the tension of the spring in valve 92 and increase the tension of the spring in valve 93, thereby decreasing the pressure required to open valve 92 and increasing the pressure required to open valve 93. Therefore, the sum of the resistances of the two valves remains constant.

In order that the springs of resistance valves 92 and 93 may be individually adjusted, member 94 is shown in Fig. 6 as being provided with two separate adjusting screws 25 and 26 which are threaded`, respectively, through the casings of valves 92 and 93 and into contact with the springs therein. ,l

Screw 25'has a nut 21 fixed thereon near its outer or upper end and that part of the screw beyond nut 21 is unthreaded and fitted in the bore of a hollow nut 28 which isfixed upon the outer or lower end of' screw 26. Nuts 21 and 28 may be fixed for simultaneous rotation by means of a set screw 29 threaded through the wall of nut 28 into contact with the unthreaded portion of screw 25.

When screw 29 is loosened, adjusting screws 25 and 26 may be turnedindependently of each other to individually adjust the resistances of valves 92 and 93, respectively.` Then by tightening set screw 29, nuts 21 and 28 will be fastened together so that rotation of one will cause rotation of theother and thereby cause the resist- .ances of valves 92 and 93 to be simultaneously The inlets of valves 92 and 93 are connected to valve 9| by channels to be presently described, the outlet of valve 92 is connected to a channel 95, and the outlet of valve 93 is connected to channel 95 by a channel 96.

In order that hydraulic motor 10-1| may be operated at predetermined speeds, reversing valve 15 functions as a follow-up valve under the joint control of piston 1| and a constant speed element, as by means of a differential |00 having one leg connected to piston 1|, its second leg connected to a constant speed element and its third leg connected to plunger 16 of valve 15.

As shown, differential |00 is provided with a casing |0| and two shafts |02 and |03 which represent the three legs of the differential. Casing 0| has a ring gear |04 iixed thereon and in mesh with a rack |05 which is connected by a rod |06 to an arm |01 fastened to piston rod 12. Shaft |02 is connected through a reduction gear |08 to a. reversible constant speed electric motor |09, and shaft |03 hask fastened thereon a gear ||0 which meshes4 with a rack connected to the stem of, reversing valve plunger 16.

The arrangement is such that, when pump 30 is operating, electric motor |09 when energized will operate differential |00 to move reversing valve plunger 16 in a direction to permit liquid to enter cylinder 10 and move piston 1| which, through rod 12, arm |01, rod |00 and rack |05 will operate differential |00 to -move reversing valve plunger 16 in a direction to stop delivery of liquid to cylinder 10. Consequently, if piton 1| tends to move in either direction faster than the rate prescribed by the speed of electric motor |09, it will automatically operate valve-10 to reduce the rate of delivery of liquid to cylinder 10 and thereby reduce the speed of piston 1|.

Conversely, it piston 1| should move slower than the prescribed speed, it would permit electric motor |09 to 'operate dierential |00 to open valve 15 wider so that more liquid could flow to cylinder 10 and thereby accelerate piston 1|. The speed of electric motor |09 thus determines the speed of piston 1| at all times regardless of the direction of movement. i

The speed of piston 1| may be varied by varying the speed ratio between electric motor |09 and 'rack This may be accomplished by either-varying the speed of electric motor |09 or by varyingv the gear ratio between electric motor" 09 and rack as by varying the ratio of reduction gear |08. Therefore, if the speed ratio between electric motor |09 and rack |I| is the same inall sections of the press, all of the hydraulic motors 10-1I will be operated at the same speed and, if this speed ratio is different in diiferent sections, the hydraulic motors 10-1l will operate at different speeds but those speeds will remain proportional to each other.

Electric motor |09 is adapted to be reversed by means of reversing switch 52 and itis adapted to be started and stopped by means of a magnetic switch ||5 through which it is connected, by means of conductors arranged in a. conduit H6, to the conductors arranged in conduit 55.

Magnetic switch ||5 has its operating mechanism connected, by means of conductors arranged in a conduit ||1, to a pilot switch I8 which is arranged at the end of valve 9| and has its operating member ||9 connected to the plunger |20 thereof so that switch ||8 will be opened or closed when valve 9| is operated.

Valve plunger |20 is provided with three spaced apart heads or pistons |25, |26 and |21 which are closely fitted in the bore oi the valve casing to control communication between five ports |29," |29, |30, |3| and |32 which are formed in the wall of the bore.

Port |28 is connected by a channel |34 to the upper end of cylinder 10,4 port |29 is connected by a channel |35 to the inlet of resistance valve 93, port |30 is connected by a channel |36 to the inlet of resistance valve 92, port |3| has channel connected thereto, and port 32 is connected by a channel |31 to channel |34 intermediate the ends thereof. Channel is also connected to channel |34 intermediate the ends thereof.

The casing of valve 9| is connected at the extreme lower end of its bore to one end of a channel |38 and at the upper end of its bore to one end of a channel |39 the other end of which is connected to return channel 45 intermediate the ends thereof. Channel |38 is connected to return channel 45 through a check valve |40 and to channel 90 through a check valve 4| and a resistance valve |42. Check valve |40 permits liquid to be drawn into channel |38 from return Same.

'except through resistance valve |42 which is adjusted to open at a predetermined pressure.

The arrangement is such that, when the -pressure in channel 88 exceeds a predetermined value, liquid will breakthrough resistance valve |42, ilow through channel |38 to the lower end of the valve casing and exert sufficient force on the lower face of piston |21 to move valve |20 upward against the resistance of a spring |43 which is arranged in the upper end of the valve casing to move valve |20 downwardwhen the pressure below piston |21 drops to a low value.

Operation Assuming that the speed ratio between electric motor |08 and rack is the same in all press sections, that pump |30 and electric motors |08 are operating and that the several parts are in the positions shown in Fig. 2, the press will operate as follows:

Pump 30 will deliver liquid into channel 42 and this liquid will iiow to each section through channels 42 and 84, valve 15, channel 90, valve8| and channels |31 and |34 to the upper end of cylinder and force piston 1| downward. Piston 1|' will eject liquid from the lower end oi cylinder 10 through channel 88, resistance valve 81, channel 85, valve and channels'18 and 45 into reservoir 44.

In each section, electric motor |09 will tend to open valve 15 wider and piston 1| will tend to close it, as previously explained, so that the opening through valve 15 remains substantially the Valve 15 thus throttles the ilow of liquid to cylinder 10 and permits only suillcient liquid to flow therethrough to move piston 1| at the desired speed.` Consequently, all of the hydraulic motors 101| are operated simultaneously and the operation of one is not aifected by the operation of any other.

When further downward movement of platen 13 is arrested by abutment 14 and piston 1| is causing platen 13 to exert a predetermined force against abutment 14 or against any material therebetween, the pressure in channel 80 will rise sufficiently to permit nquld to break through resistance valve |42 and then flow through channel |38 to the lower end of valve 8| and raise its plunger to the position shown in Fig. 3.

As plunger |20 moves upward, it will eject liquid from the upper end of the valve casing through channel |39 into return channel 45 and near its upper limit it will operate pilot switch I8 which will cause magnetic switch ||5 to open and stop motor |09, thereby causing reversing valve plunger 1B to remain stationary for the reason that piston 1| has stalled. At the same' time, plunger |20 will close port |32 and open ports |28 and |30.

Closing port |32 and opening port |30 will prevent the pressure in channel 90 from extending to the upper end of cylinder 10 except through channel |36 and resistance valve 82. Since there is no flow of liquid through valve 15 at this time, the throttling eifect of valve 15 will be eliminated and the pressure in channels 90 and |36 will rise until it equals the pressure in supply channel 42.

As the pressure in channel |38 rises, resistance valve 92 will open and pressure will extend therethrough and through channels 85 and |34 Ito the upper end of cylinder 10, thereby causing to be exerted upon piston 1| a pressure equal to the diierence between the pressure in channel 42 and the pressure required to Iopen resistance valve 82.

If the pressure in channel 42 should increase sumciently or if the upward force acting on pressure member 13 should increase and thereby increase the pressure in the upper end of cylinder 10 above the predetermined value, resistance valve 83 wouldV open and permit liquid to escape from cylinder 10 through channel |34, valve 8|, channel |35, resistance valve 83, channels 88 and 85, valve 15 and channel 18 into return channel 45 until the pressure in cylinder 10 was reduced to the predetermined value.

Valve 15 may be open wider at this time than when piston 1| is moving downward due to a very slight delay between the stalling of piston 1| land the stopping of motor |08 but such a widening of the valve opening will have no eiYect upon the operation of any other press section for the reason that only enough liquid may be delivered to cylinder 10 to maintain therein a pressure equal to the difference between pump pressure and the pressure required to open resistance valve 82. Consequently, if one hydraulic motor stalls before am'7 of the other hydraulic motors stall, such other motors will continue to operate at their predetermined speeds until they stall and then the control mechanisms thereof will operate to enable pump to maintain individual holding pressures in the cylinders 10 thereof.

As previously explained, pump control 38 causes pump 30 to deliver liquid at a maximum rate until pump pressure reaches a predetermined maximum and to then reduce its displacement until it is delivering just suilicient liquid to maintain that maximum pressure constant, that the 4resistance valve 82 of each section is so adjusted that its closing pressure is equal to the difference between pump pressure and the maximum pressure required in the cylinder 10 of that section, and that the relief valve 93 in each section is adjusted to open at a pressure higher than the maximum pressure required in the cylinder 10 of that section, it being obvious that pump 30 cannot create a pressure greater than the pressure required to open both of valves 82 and 83 in any one section.

For example, if pump pressure is 1000# per square inch and it is desired that the pressure created inthe cylinder 10 o f one section be limited to 900# per square inch, the valve 82 in that section is adjusted to have a closing pressure of 100# per square inch and the associated valve 93 is adjusted to open at about 910# per square inch. If the valves 92 and 93 of a second section are adjusted to have resistances of 200 and 810# per square inch respectively, pump 30 will create a pressure of 800# per square inch in the cylinder 10 of the second section without aiecting the pressure in the cylinder 10 of any other section. If the valves 82 and 83 of a third section are adjusted to have resistances of 400 and 900# per square inch respectively, pump 30 will create a maximum pressure of 600# per square inch in the cylinder 10 of the third section and the piston 1| thereof will be able to resist an upward force 50% greater than the downward force exerted thereon by the liquid.

When it is desired to raise the platens, the operator will swing hand lever 50 toward the left and thereby move valve plunger 53 from the position shown in Fig. 2 to the position shown in Fig. 4. Plunger 53 will move rapidly until it opens communication between channels 5| and 52 and then dashpot plunger 64 will enter dashpot of the valve casing through channel |38 and check valve |4| into channel 90 and, as it approaches the limit of its downward movement, itv operates pilot switch ||8 which causes magnetic switch l5 to close so that motor |09 will be energized when switch 52 is closed.

When lever 50 -has been swung to the position shown in Fig. 4, valve plunger 53 will 4have closed communication between channels 8| and 62, so that pump 30 can create pressure in channel 42, and then will have closed switch 52 so vthat motors |09 are energized but are operatein a direction y opposite to that in which they were operating when pistons 1| were being moved downward.

In each section, motor |09 will move reversing valve plunger 16 downward to open port 8| to port -83 and to open port 82 to channel 80 as shown in Fig. 4. Then liquid from channel 42 may ow through channel 84, valve 15, channel 85, check Valve 88 and channel 88 to the lower end of cylinder 10 and move piston 1| upward. Piston 1| will eject liquid from the upper end of cylinder 10 through channels |34 and |31, valve 9|, channel 90, valve 15 and channels 80 and 45 into reservoir 44. y l

If all of the pistons 1| should rise high enough to stall, pump pressure would rise to the predetermined maximum and pump 30 would reduce its stroke until it is delivering Ajust suilicient liquid to maintain that maximum pressure constant. However, before pistons 1| stali, the operator may swing lever 50 to the position shown in Fig. 5 to open channel to channel 62 and then the pump would be bypassed and the press remain idle. v

The operator may then restart the press by swinging lever 50 to the position shown in Fig. 2 in which position channel 62 is blocked, so that pump 30 can create pressure in channel 42, and switch 52 is closed so that motors |09 are caused to operate in a direction to move valve plunger 18 upward to admit liquid to the upper ends of cylinders 10. Pistons 1| will thenV move downward and move valve plunger 16 downward until 65'?i ttedvin said cylinder, a valve for directing liquid from said source to one end or the other of said cylinder and for controlling the rate at which liquid is delivered thereto, a standard speed element, means responsive to any variation in the speed of said piston relative to the speed of vsaid element for adjusting said valve to vary the ow of liquid to said cylinder and thereby correct such variation in the speed of said piston, means for maintaining the working pressure in said cylinder Within narrow limits, and means responsive to th working pressure in said cylinder reaching a predetermined maximum for rendering the last mentioned means effective and simultaneously stopping said standard speed element.

2. A hydraulic press, comprising a source of motive liquid having a pressure above a predetermined value and a plurality of press units operated by liquid from said source, each of said units including a stationary cylinder, a piston fitted in said cylinder, a valve for directing' liquid from saidy source to one end or the other of said cylinder and for controlling the rate at which liquid is delivered thereto, a standard speed ele'- ment, means responsive to any variationin the speed of said piston relative to the speed of said predetermined maximum and to cause a drop in pressure between said cylinder and said valve, a second valve for controlling saidpressure control means and normally'rendering said pressure control means ineiective, and means responsive to the working pressure in said cylinder reaching a predetermined maximum for shifting said second valve to thereb'y render said pressure control means effective.

3. A hydraulic press, comprising a source of motive yliquid having a pressure above a predetermined value and a plurality of press units operated by liquid from said source, each of said units irleluding a stationary cylinder, a piston tted in said cylinder, a valve for directing liquid from said source to one end or the other of said cylinder and for controlling the rate at which liquid is delivered thereto, astandard speed element, means responsive to any variation in the speed of said piston relative tothe speed of said element for adjusting said valve to vary the flow of liquid to said cylinder and thereby correct such variation in the speed" of said piston, pressure control means adapted when eie'ctive to prevent the pressure in said cylinder exceeding a predetermined maximum and to cause a drop in pressure between said cylinder and said valve, a second valvel for controlling said pressure control means and normally rendering said pressure control means ineffective, means responsive to the working pressure in said cylinder reaching a prev determined maximum for shifting said second valve to thereby render said pressure control means effective, and means operated substantially in unison with said second valve for stopping said standard speed element.

4. A hydraulic press, comprising a source of motive liquid having a pressure above a predetermined value, a plurality of press units operated by liquid from such source; eachl of said units including a stationary cylinder, a piston tted in said cylinder, a valve'for directing liquid to opposite ends of said cylinder alternatively to advance and retract said piston and for controlling the rate at which liquid is delivered to said cylinder to thereby control the speed of said piston, a standard speed element, means responsive to any variation in the speed of said piston relative to the speed of said element foiadjusting said valve to vary the flow of liquid to said cylinder and thereby correct the variation in piston speed, pressure control means adapted when effective to prevent the pressure in said cylinder exceeding a predetermined maximum and to cause a drop inpressure between said cylinder and said valve.asecondvalvei'orcontrollingsaidpres sure control means and spring biased to a position to render said pressure control means in- A means for reducing the pressure at said source to thereby permit said second valve to shift to a position to render the pressure control means ln effective, means for reversing the standard speed elements in all of said units, and means for operatingsaid pressure reducing means and said reversing means simultaneously.

5. A hydraulic press, comprising a source oi motive liquid having a pressure above a predetermined value, a plurality of press umts operated by liquid from such source; each of said units including a stationary cylinder, a piston ntted in said cylinder, a valve for directing liquid to opposite ends of said cylinder alternatively to advance and retract said piston andi'or controlling the rate at which liquid is delivered to said cylinder to thereby control the speed of said piston, a standard speed element, means responsive to any variation in the speed of said piston relative to the speed of said element for' adjusting said valve to vary the ilow of liquid to said cylinder and thereby correct the variation in piston speed, pressure control means adapted when elective to prevent the pressure in said cylinder exceeding a predetermined maximum and to cause adrop in pressure between said cylinder and said -valve,

a second valve for controlling said pressure control means and spring biased to a position to render said pressure control means ineifective. means responsive to the working pressure in said cylinder reaching a predetermined maximum for shifting said second valve to render said pressure control means effective, and means operative in unison with said second valve for stopping said standard speed element; means for reducing the pressure at said source to thereby 'permit said second valve to shift to a position to render said f pressure control means ineil'ective, means for reversing the standard speed elements in all of said units, and means for operating said pressure reducing means and said reversing means simultaneously.

6. A hydraulic press, comprising a source of motive liquid having a pressure a-bove a predetermined value, a plurality of press units operated by liquid from such source; each of said units including a stationary cylinder, a piston iltted in said cylinder, a valve for'directing liquid to opposite ends of said cylinder alternatively to advancev and retract said piston and for controlling the rate at which liquid is delivered to said cylinder to thereby control the speed of said piston. an electric standard speed element, means responsive to any variation in the speed of said piston relative to the speed of said element for adjusting said valve to vary the flow of liquid to said cylinvder and thereby correct the'variation in piston speed, pressure control means adapted when effective to prevent the pressure in said cylinder exceeding a predetermined maximum and to cause a drop in pressure between said cylinder and said valve, a second valve for controlling said pressure control means and spring biased to a position to render said pressure control means "ineffective, means responsive to the working pressure in said cylinder reaching a predetermined maximum for shifting said second valve to render said pressure control means eii'ective, a magnetic switch for continuing said speed element, and a pilot switch operated in response to movement of said second valve for causing said magnetic switch to stop said element upon said second valve rendering said pressure control means ineffective; means for reducing thepressure at said source to thereby permit said second valve to shift to a position to render said pressure control means ineffective, a reversing switch for reversing the standard speed elements in all said units, and means for operating said pressure reducing means and said reversing switch simultaneously.

7. A hold-down system, comprising a source of motiv liquid having a pressure abovela predetermined'value and a plurality oi.' hydraulically actuated hold-down units operated by liquid supplied thereto from said source, each of said units including a cylinder having a power end and a retraction end, a piston iltted inl said cylinder, a ilrst control valve for directing liquid to opposite ends of said cylinder alternatively to advance and retract said piston and for controlling the rate at which liquid is delivered to said cylinder to control the speed of said piston, a substantially constant speed element, a diiferential having one leg thereof driven from said constant speed element and a second leg thereof driven by motion imparted thereto from said piston whereby any variation in the speed of said piston relative to the speed of said element causes rotation of the third leg of said diiferential, means responsive to rotation of the third leg of said diiferential for shifting said valve to vary the rate' of delivery of liquid to said cylinder to thereby maintain the speed of said piston proportional to the speed of said element, a resistance valve connected to the power end of said cylinder for causing a drop in the pressure of the liquid delivered thereto, a relief valve for limiting' the pressure in the power end of said cylinder, and a second control valve operable either to permit the liquid passing through said ilrst control valve to ilow freely to the power end of said cylinder or to direct said liquid thereto through said resistance valve.

8. A hold-down system, comprising a source of motive liquid having a pressure above a predetermined value and a plurality of hydraulically actuated hold-down units operated by liquid supplied thereto from said source, each of said units including a cylinder having a power end and a retraction end, a piston iltted in said cylinder. a ilrst control valve for directing liquid to opposite ends of said cylinder alternatively to advance and retract said piston and for controlling the rate at which liquid is delivered to said cylinder to control the speed of said piston, a substantially constant speed element.. a differential having one leg thereof driven from said constant speed element and a second leg thereof driven by motion imparted thereto from said piston whereby any variation in the speed of said piston relative to the speed of said element causes rotation of the third leg of said diiferential, means responsive to rotation of the third-leg of said dierential for shifting said valve to vary the rate of delivery of liquid to said cylinder to thereby maintain the speed of said piston proportional to the speed of said element, a resistance valve connected to the power end of said cylinder for causing a drop in the pressure of the liquid delivered thereto, a relief valve for limiting the pressure in the power end of said cylinder, a second control valve operable either to permit the liquid passing through said ilrst control valve to ilow freely to the power end oi' said cylinder or to direct said liquid thereto through said resistance valve, and means for reversesaid piston. yfr! I 'l 9. A holdfdown ys'ystem,:comprising a source of motive-liquid having a pressureabove a predetermined value and. a pluralityoi hydraulically actuated'hold-down units operated by liquid supplied thereto from said source, each of said units including a cylinder having a 4power end` and a retraction end,v a piston fitted in said cylinder, a first control valve for directing liquid to opposite ends of said cylinder alternatively to advance and retract said piston and for controlling the rate at which liquid is delivered to said cylinder to control the speed of said piston, a substantially constant speed element, a differential having one leg thereof driven from said constant speed element and a second legthereof driven by motion imparted lthereto from said piston whereby any variation in the speed of lsaid piston relative to the speed of said element causes-rotation of the third leg of said diierentiahmeans responsive to rotation of the third leg of said differential for shifting said vvalvel to vary the rate of delivery of liquid to said ycylinder to thereby maintain the speed of said/piston proportional to the speed of said element, a resistance Valve connected to the power end of said cylinder for causing a drop in the ,pressure of the liquid delivered thereto, a relief valve for limiting the pressure in the power end of said cylinder, a second control valveoperable either to 'permit the liquid passing through said first control valve to flow freely to the power end of said cylinder or to direct said liquid thereto through said resistance valve, and -means responsive to the stalling of said piston for stopping said constant speed element.

n 10. A hold-down system, comprising a source of motive liquid having a pressure above a predetermined value and a plurality of hydraulically actuated hold-down units operated by liquid supplied thereto from said source, each of said units including a cylinder having a power end and a retraction end, a piston fitted in said cylinder, a first control valve for directing liquid to opposite ends of said cylinder alternatively to advance and retract said piston and for controlling the rate at which liquid is delivered to said cylinder to control the speed of said piston, a substantially constant speed element, a differential having one leg thereof driven from said constant speed element and a second leg thereof driven by motion imparted thereto from said piston whereby any variation in the speed of said piston relative to the speed-of said element causes rotation of the third leg of said differential, means responsive to rotation of the third leg of said differential for shifting said valve to vary the rate of delivery of liquid to said cylinder-to thereby maintain the speed of said piston proportional to the speed of said element, a resistance valve connected to the power end of said cylinder for caus- .ing a drop in the pressure ofthe liquid delivered thereto, a relief valve for limiting the pressure in the power end of said cylinder, a second control valve operable either topermit the liquid passing through said rst control valve to flow freely to the power end of said cylinder or to direct said liquid thereto through said resistance valve, means for reversing said constant speed element to thereby reverse said piston, and means responsive to the stalling of said piston for stopping said element.

11. A hold-down system, comprising a source of motive liquid having a pressure above a predetermined value and a plurality of hydraulically actuated hold-down units operated by liquid sup'- plied thereto from said source, each of said units including a cylinder having a powerv end and a retraction end, a piston fitted in said cylinder, a

first control valve for directing liquid to oppo site ends of said cylinder alternatively to.advance and retract said piston and for controlling the rate at which liquid xisdelivered to said cylinder to control the speed of said piston, a substantially constant speed element, a diierential having one leg thereof driven from said constant speed element and a second leg thereof driven by motion imparted thereto from said piston whereby any variation in the speed of said piston relative to the speed of said element causes rotation of the third leg of said differential. means responsive to rotation of the third leg of said differential for shifting said valve to vary the rate of delivery of liquid to said cylinder to thereby maintain the speed of said piston proportional to the speed of said element, a resistance valve connected to the power end of said cylinder for causing a drop in the pressure of the liquid delivered thereto, a relief valve for limiting the pressure in the power end of said cylinder, a second valve for controlling the delivery of liquid to the power end of said cylinder and normally permitting the liquid passing through said first control valve to iiow freely thereto, and means responsive to said piston stalling at the end of its advance movement for operating said second control valve to cause it to direct said liquid to the power end of said cylinder through said resistance valve.

l2. A hold-down system, comprising a source of motive liquid having a pressure above a predetermined Value and a plurality of hydraulically actuated hold-down units operated by liquid supplied thereto from said source, each of said units including a cylinder having a power end and a retraction end, a piston fitted in said cylinder, a rst control valve for directing liquid to opposite ends of said cylinder alternatively to advance and leg thereof driven from said constant speed element and a second leg thereof driven by motion imparted thereto from said piston whereby any variation in the speed of said piston relative to the speed of s aid element causes rotation of the third leg of said differential, means responsive to rotation of the third leg of said differential for shifting said valve to vary the rate of delivery of liquid to said cylinder to thereby maintain the speed of said piston proportional tothe speed of said element, means for reversing said constant speed element to thereby reverse said piston, a. resistance valve connected to the power end of said cylinder for causing a drop in the pressure of lthe liquid delivered thereto, a relief valve for limiting the pressure in the power end of said cylinder, a second valve for controlling the delivery of liquid to the power end of said cylinder and normally permitting the liquid passing through said first control valve to ow freely thereto, and means responsive to said piston stalling at the end of its advance movement for termined value and a plurality oi' hydraulically actuated hold-down units operated by liquid supplied'thereto from said source, each of said units imparted thereto from said piston whereby any variation in the speed of said piston relative to the speed of said element causes rotation of the third leg of said dierential, means responsive to rotation of the third leg of said differential for shifting said valve to vary the rate of delivery of liquid to said cylinder to thereby maintain the speed of said piston proportional to the speed of said element, means responsive. to the stalling of said piston for stopping said constant speed element, a resistance valve connected to the power end of said cylinder for causing a drop in the pressure of the liquid delivered thereto, a relief valve for limiting the pressure in the power end of said cylinder, a second valve for controlling the delivery of liquid to the power vend of said cylinder and normally permitting the liquid passing through said first control valve to ow freely thereto, and means responsive to said piston stalling at the end of its advance movement for operating said second control valve to cause it to direct said liquid to the power end of said cylinder through'said resistance valve.

14. A hold-down system, comprising a source of motive liquid having a pressure above a predetermined value and a plurality oi.' hydraulically actuated hold-down units operated by liquid supplied thereto from said source, each of said units including a cylinder having a 'power end and a retraction end, a piston fitted in said cylinder, a rst control valve for directing liquid to opposite ends of said cylinder alternatively to advance and retract said piston and for controlling the rate' at which liquid is delivered to said cylinder to control the speed of said piston, a substantially constant speed'element, a differential having one leg thereof driven from said constant speed element anda second leg thereof driven by motion imparted thereto from said piston whereby any variation in the speed of said piston relative to the speed of said element causes rotation of the `n third leg of said differential, means responsive of said, element. means forreversing said conthrough said first control valve to flow freely thereto, and means responsive to said piston stalling at.the end of its advance movement i'orv operating said second control valve to cause it to direct said-liquid to the power end of said cylinder through said resistance valve.

15.- A hold-down system, comprising a pump adapted to deliver motive liquid at a pressure above a predetermined value and a plurality of hydraulically actuated hold-down umts operated by liquid supplied thereto from said pump, each of said units including a 'cylinder having a power end and a retraction end, a piston fitted in said cylinder, a first control valve for directing liquid to opposite ends of said cylinder alternatively to advance and retract said piston and for controlling the rate at which'liquid is delivered to said cylinder to control the speed of said piston, a substantially constant speed element, a differential having one leg thereof driven from said constant speed element and a second leg thereof driven by motion imparted thereto from said piston whereby any variation in the speed of said piston relative to the speed of said element causes rotation of the third leg of said differential, means responsive to rotation of the third leg of said differential for shifting said valve to vary the rate of delivery of liquid to said cylinder to thereby maintain the speed of said piston proportional to the speed of said element, a resistance valve connected to the power end of said cylinder for causing a drop in the .pressure of the liquid delivered thereto, a relief valve for limiting the pressure in the power end of said cylinder,

-a second control valve operable either to permit v operating said second control valve to cause it to direct liquid to the power end of said cylinder through said resistance valve, means for reducing pump pressure to a low value, and means operable in unison with said pressure reducing means for reversing the constant speed elements of all of said units simultaneously.

WALTER FERRIS. 

